Many lymphokines of particular interest in tumor immunology are secreted by lymphocytes that have trafficed through the thymus gland. The exact nature of maturational steps and the thymic polypeptides that control these steps are not known. Although much progress has been made in the separation and purification of thymic polypeptides, little is still known about which of these thymic polypeptides act on lymphocyte maturation, act on lymphokine production or are lymphokines themselves. Two thymosin polypeptides have shown distinct properties in the production or action of lymphokines. Thymosin Alpha 1 (28 amino acids, mol. wt. of 3108 and a pI of 4.2) has been shown to induce the capability of antigenreactive peripheral blood lymphocytes of thymectomized guinea pigs to produce macrophage Migration Inhibition Factor (MIF). This has been shown both by the agarose droplet MIF assay and by the agarose droplet MIF assay. Another polypeptide, thymosin Beta 4 (43 amino acids, mol. wt. of 4982 and a pI of 5.1) mimics MIF or is the active component of MIF in that it inhibits macrophage migration itself in a non-cytotoxic manner.